1. Field of the Invention
The present invention relates to an optical fiber amplifier to be applied to realize larger transmission capacity and longer distance transmission of a WDM transmission system.
2. Description of the Related Art
In recent years, with rapid progress of Internet technology, the demand for information has increased drastically, and further improvement in capacity and formation of a more flexible network are requested in the main line optical transmission system to which information capacity is concentrated.
The wavelength multiplexed (WDM) transmission system is the most effective system at present for such system demand, and it is now being investigated for commercial use mainly in the USA.
An amplifier for amplifying light using a rare earth element doped optical fiber, for example, an Er-doped fiber optical amplifier (Erbium-doped fiber amplifier: EDFA) is a key component to realize the wavelength multiplexed optical transmission system because it can amplify the waveform multiplexed light signals using its wider gain bandwidth.
It is known that EDFA can cover not only the amplifying bandwidth (1530 to 1565 nm), called the conventional band (C-band), which has mainly been used, but also the amplifying bandwidth (1570 to 1605 nm), which in recent years has been called the long wavelength band (L-band).
In the current EDFA system, it is possible to realize wavelength multiplexed amplification of about 200 wavelengths within the band combining the C-band and L-band.
In the EDFA system, it is required to select the inversion population ratio that provides the condition of the gain for each wavelength multiplexed signal light such that this gain becomes constant in each application wavelength band.
FIG. 1 illustrates the dependence on wavelength of the gain coefficient of a unit length of Er-doped fiber (EDF) when the inversion population ratio is changed. The lowest gain characteristic in the figure indicates the non-inverted distribution condition (inversion population ratio=0), while the highest gain characteristic indicates the condition (inversion population ratio 1) of complete inverted distribution (pumped to upper level). The intermediate characteristics indicate the condition where the inversion population ratio is increased in increments of 0.1.
FIG. 1 suggests: (1) the C-band EDFA operates at a wavelength near the center wavelength (1530 nm) for radiation/absorption of Er ions and can obtain sufficient gain using a short EDF (Er-doped fiber); and (2) the L-band EDFA has a small gain coefficient per unit length and requires a long-length EDF in comparison with the C-band EDFA.
FIG. 2 illustrates a basic structure of EDFA for L-band optical amplification. FIG. 2 includes EDF 1, optical isolators 2-1 and 2-2, a wavelength multiplex coupler (WDM coupler) 3, and a semiconductor laser 4 to pump laser light.
The wavelength multiplexed signal light supplied from the transmission path is supplied to the EDF 1 via optical isolator 2-1 and WDM coupler 3. The EDF 1 supplies almost identical gain to the L-band signal light as the C-Band and is therefore required to have a long length to attain the gain identical to that of the signal light of the C-band. The output of the EDF1 is provided via the optical isolator 2-2.
However, in a the gain-shift type optical amplifier such as the L-band EDFA, in which a low inversion population ratio is necessary to obtain the flat gain for the target amplifying wavelength band, and a rare earth element doped fiber is required to have a long-length to obtain the necessary gain, it is difficult to realize stable and efficient amplification.